1. Field of the Invention
This invention relates to a focusing screen disposed on the observation optical path of an optical apparatus such as a camera, and in particular to a focusing screen in a split image type range finder.
2. Description of the Prior Art
A focusing screen having a pair of wedge prisms disposed on the center of the focal plane to enhance the accuracy of focusing has heretofore been used as the focusing screen of a camera. The wedge prisms are of a semicircular shape and disposed so as to deflect light rays in opposite directions, and the two wedge prisms together constitute a circular focus detecting portion. Light rays condensed from an objective lens onto the center of the focusing screen are refracted by the pair of wedge prisms on the central portion of the focusing screen and arrive at an eyepiece along the optic axis. The image of the field of view of the focus detecting portion when the focus is not coincident is separated at the boundary between the two wedge prisms in the direction of the boundary lines. When the focus is coincident, the image is not separated but observed.
However, in such a split image coincidence type focusing screen, there has been a disadvantage that when the stop is stopped down or relative to a dark lens of great F-number, the surfaces of the prism are dark-changed to make focus detection difficult. Enhancing the accuracy of focus detection and preventing a dark change of the surfaces of the prism are in a contrary relation with each other and it is difficult to satisfy the two at a time, and various contrivances to this problem have long been proposed.
As an example, there is disclosed in Japanese Patent Publication No. 9531/1962 a focusing screen in which the surfaces of a split prism are made into fine particulate surfaces and by the scattering of light thereby, the dark change in the case of a small stop is prevented. In this focusing screen, however, during the non-focusing, the image of the split prism portion creates blur similar to the mat surface around it and therefore, focusing is effected with the aid of the blur rather than with the aid of the displacement of the image, and it has been difficult to accomplish focusing of good accuracy intrinsic to the split prism. In this focusing screen, the scattered lights on the surfaces of the split prism have been substantially isotropic and created in all directions and therefore have been almost useless, and the focus detecting portion has become dark and focusing has been difficult to accomplish.
Also, examples of the focusing screen in which a diffraction grating structure is formed on the surface of a split prism are known from Japanese Laid-open Patent Application No. 626/1979 and U.S. Pat. No. 4,338,012. These constructions are considerably excellent as compared with the aforedescribed construction, but due to their diffraction grating structure the lines caused by the darkening of the grooves and valleys of the diffraction grating which extend in a direction perpendicular to the boundary line of the prism become readily conspicuous and an eyesore, and also due to the periodical structure of the diffraction grating, the directions of dispersion of light become separate, and the object image in the case of out-of-focus has become multi-line blurred and has not always been sightly. Also, such constructions have suffered from a problem that if there is a periodical structure in the object image, moire stripes are created to make the object image more unsightly. Further, there has been a disadvantage that due to the action of the diffraction grating, coloring of the blurred image appears when the stop is slightly stopped down.
Also, constructions in which the focus detecting portion is constructed of only a diffraction grating instead of a split prism and each element of the grating is blazed and a plurality of blaze angles are provided to thereby diffract light in many directions and prevent the phenomenon of dark change are disclosed in Japanese Patent Publication No. 29123/1964 and the aforementioned U.S. Pat. No. 4,338,012. However, these constructions have a nature peculiar to the diffraction grating more strongly than the aforedescribed construction in which a diffraction grating is formed on the surface of the split prism and therefore, in these constructions, the above-noted disadvantages have been emphasized and the disadvantage that in the case of stop-down, remarkable coloring occurs to the blurred image of the object to be photographed has been unavoidable.